For pico projectors that are used in portable devices like cell phones or other devices, power dissipation is important. Pico projectors may be of any type such as liquid crystal on silicon (LCOS), digital light processing (DLP), and scanning-mirror system (March of the Pico Projectors, IEEE Spectrum, May 2010) or other similar devices. In all of the systems, there is a light source which may use either LEDs or LDs (laser Diodes). Lighting LEDs or LDs and these light sources usually may be the most power hungry blocks of a pico projector.
Traditionally, a light source may be driven by using a DC/DC converter, and there are different types of DC/DC converters being used such as Buck, Boost or Buck-Boost converters. One of the widely used is the Buck-Boost converter. Such a circuit is shown in the FIG. 1.
The Buck Boost converter may include an error amplifier, a pulse width modulator (PWM), an off-chip inductor and a divider network. The principle of the Buck Boost converter is well known. Basically, the battery charges the inductor on clock phase Φ1, and on clock phase Φ2, the energy is transferred from the inductor L to the capacitor C (Notice that the switching sequence can be different depending on the Buck, Boost, or Buck-boost type of the converter.). The external resistor ladder determines the output voltage by a feedback that consists of an error amplifier and a PWM. Due to the feedback, we have:Vout X(R2)/(R1+R2)=Vref
The light source is usually current driven. The voltage across the light source differs for different types of light sources. A current sink that is controlled by a digital to analog converter (DAC) is usually used to regulate the current into the light source.
The minimum output voltage has to be larger than the voltage drop across the light source and that across the current source. It is obvious that the power efficiency of the device suffers due to the voltage needed for the current source. Another issue is to share single DC/DC converter to drive several LEDs or LDs (e.g., for red, blue, and green). Those LEDs or LDs have different voltage drops. If the DC/DC converter output voltage is set too high, the efficiency drops significantly when driving the light source having a lower voltage drop. Techniques may be used to dynamically adjust the DC/DC converter output voltage according to the light source voltage drop by changing the feedback ratio around the DC-DC converter. The problem with these techniques is the large ringing and the long settling time in the DC/DC converter when we adjust the output voltage. Large settling time results in the loss of efficiency. The color accuracy also suffers from the large rise/fall time as the current source (sink) has a finite output impedance and while settling. Due to the finite output impedance in the current sink, the current into the light source changes as the voltage drop is changing. Of course, this still cannot solve the efficiency issue related with the voltage headroom of the current source even after settling.
Even if only one single DC-DC converter is dedicated to the Light Source, The voltage drop across the Light Source is a function of its current, so it is changing when the current changes. So we have to adjust the output of the DC-DC converter accordingly, to minimize the voltage drop, hence saving power. Or if the DC-DC converter output is constant, lower efficiency results due to extra voltage on the current driver which is not needed.